I'm knocking the wall down between my bathroom and seperate toilet, where the toilet used to be i'll be putting in a walk in shower. How easy will it be to connect the shower waste to the old toilet waste pipe? can it be done? will the shower waste need some kind of trap to stop smells?

If I have understood you correctly, yes it can be done with a reducing kit,go to a good plumbers merchant -not b & crew.
Fairly easy to do, you should have a shower trap fitted to stop smells and comply with the regs.